The present invention relates to a surface defect detecting apparatus for use with metallic materials having a circular external shape, such as, steel round billets, round bars or steel pipes.
Among the automatic flaw detecting apparatus heretofore known in the art for detecting the surface defects of round billets, are those which are called as magnetic flaw detecting apparatus. Though this type of flaw detecting apparatus has the advantage of automatic flaw detection over the known magnetic particle flaw detection method, there is a disadvantage that since magnetic flux is passed from an electromagnet through a rotating round billet and the resulting leakage flux from a defective area is detected by sensors such as magnetism responsive diodes, the sensors must be arranged in close proximity to the surface of the round billet with a very small gap of the order of 0.2 mm, with the result that the presence of projections or ovals on the round billet makes it impossible to effect the detection and consequently the outer surface of the billet must be preliminarily be prepared. Another disadvantage is that since each sensor is provided with a shoe which contacts with the round billet surface to maintain the very small gap, the oxide particles on the billet surface tend to deposit in the very small gap at the shoe or the gap between the diode surface and the round billet surface, thus producing ill effects on the detection function. Still another disadvantage is that since the sensors are semiconductor devices, the sensors are easily affected by heat and they are not able to detect defects in a heated round billet without cooling means. Still further disadvantage of the magnetic flux leakage detection method is that if the depth of flaws is greater than about 2 mm, for example, the amplitude of the outputs becomes constant thus making it impossible to discriminate the size of the defects, and moreover if part of the coils for applying flux to the round billet deviate from the billet end, the detection characteristic is changed and a portion of the round billet end corresponding to the diameter of the magnetic flux coil fails to be subjected to the flaw detection, the portion usually amounting as much as about 40 mm.
On the other hand, the defect areas detected automatically by a surface defect detecting apparatus of the type described above or any other suitable surface defect detecting apparatus are removed at the next processing step to produce round billets having no surface defects which in turn are used as raw material for producing a variety of products, and consequently the location of the detected defects must be accurately marked for the defect removing operation. In the past, it has been the usual practice to effect this type of marking by reinstating the recorded data of the inspection results in the round billet by an operator and consequently there is a problem of inefficiency for the operations including from the separation of reject material according to the degree of defects up to the operation of removing the defects by cutting.